The use of thermoplastic films to unitize a pallet of goods is becoming an increasingly more preferred method of bundling goods for shipment. In one method, a pallet of goods is wrapped with film by positioning the pallet load on a rotating platform. A vertical roll of plastic film adjacent the rotating platform supplies the wrapping film. The leading edge of the rolled plastic film is laid along a vertical surface of the load. As the pallet is rotated, plastic film is unrolled and drawn about the load. A braking force applied to the film roll stretches the film during the wrapping process. The tensioned, wrapped film applies a generally inward force that retains the goods in a tight bundle. At the end of the wrapping process, rotation is ceased, the film is cut and the trailing edge is tacked to the film over-wrap by tape, spray adhesive, or by the cling properties of the film itself.
Cling properties may be imparted to a non-clinging polyethylene film by addition of a tackifier such as glycerol mono or di-oleate, polypropylene glycol, sorbitan mono oleate, mono and diglycerides, fatty acid esters, and mineral or vegetable oils. Unblended thermoplastics which have an inherent cling property are also known, such as certain copolymers of polyethylene and 1-octene, 1-butene and 1-hexene. However, few of these blended or unblended films exhibit the toughness and elongation properties desirable for an industrial-use wrapping film.
It is well known in the art to combine the cling properties of the aforementioned compositions with the toughness and elongation properties of other types of polyethylene in multi-layer films. U.S. Pat. No. 4,399,180 discloses such a film having an A/B/A structure. The inner B layer is a linear low density polyethylene copolymer, and the outer A layers are a highly branched low density polyethylene. In addition to the cling property, the skins resist reduction in film width when the film is stretched.
U.S. Pat. No. 4,436,788 discloses a two layer stretch wrap film comprising a layer of ethylene vinyl acetate copolymer with a tackifier and a second layer of a LLDPE. The first layer is characterized by a comonomer incorporation of 25% or more by weight of vinyl acetate, and a melt index of about 0.1 to about 4.0. The second layer is characterized by a specific gravity of between about 0.917 and 0.945.
U.S. Pat. No. 4,367,256 discloses the use of low pressure low density polyethylene (LPLDPE) to reinforce a high pressure low density polyethylene (HPLDPE) film layer in a multi-layer film. When present in from 5-3 wt. percent, LPLDPE was found to increase the strength of a three layer film and improve the cutting characteristics of the film. The HPLDPE and LDPLDPE of that invention are characterized by melt indices of between 0.5 and 7.0 g. 10 min.sup.-1, and densities less than about 0.932 g. cm.sup.-3. A tackifier is required to impart cling properties to the film.
U.S. Pat. No. 5,019,315 discloses a three layer stretch resistant film having a good stretched cling. The skin layers of the film are made of a LLDPE copolymer having an inherent cling property. The core layer is made from a high pressure low density polyethylene and is between 15 and 35% of the overall thickness of the film. By adjusting the thickness of the core layer it is possible to make films of varying resistance to elongation.
When using these currently available three-layer films to wrap pallets, the noise associated with the wrapping process may exceed 100 Db. The noise is generated by a segment of unwrapped film extending from the pallet to the film roll. This segment of film is tensioned by the combined action of the rotating pallet and braking force applied to the film. Vibrations generated by peeling the film from the roll are transmitted to the tensioned segment and are amplified into a loud drumming sound by the vibrating film.
It is thus an object of the present invention to provide a three layer film useful for wrapping large pallets of goods and having a sound dampening property capable of reducing noise associated with the wrapping process. It is a further object of the invention to make such a film having the desirable elongation and toughness properties of a linear film.
Linear films, however, are known to undergo film width reduction, or "necking in" when stretched. Multi-layer films known to the art avoid the necking in phenomenon by including in the structure at least one layer of neck in resistant material. For instance, in U.S. Pat. No. 5,019,315, linear low density skin layers are coextruded with a neck in resistant, non-linear low density polymer which comprises the core layer. On the other hand, U.S. Pat. No. 4,399,180 discloses a film having a layer of pure LLDPE with at least one additional layer of a neck in resistant highly branched low density polyethylene.
It is a further object of the invention to counteract film width reduction without additional layers of neck in resistant material.